Offshore storage structure

ABSTRACT

LEG RETAINING WELLS ASSOCIATED WITH BOTH THE RINGWALL AND THE SECOND TANK, AND, MEANS FOR SECURING AND VERTICALLY MOVING THE LEGS RELATIVE TO THE RINGWALL AND SECOND TANK. THERE MAY ALSO BE PROVIDED MEANS ASSOCIATED WITH THE SECOND TANK FOR SECURING AND VERTICALLY MOVING IT RELATIVE TO THE SHAFT.   STABLE OFFSHORE STORAGE STRUCTURES INCLUDING: A FIRST, RINGWALL-REINFORCED TANK DISPOSALBE ADJACENT THE SEA FLOOR, A VERTICALLY EXTENDING SHAFT CENTRALLY DISPOSED WITHIN THE FIRST TANK AND EXTENDING UPWARDLY THEREFROM, A SECOND, ANNULARLY-SHAPED TANK DISPOSABLE ABOVE THE FIRST TANK, THROUGH THE CENTRAL ORIFICE OF WHICH THE SHAFT MAY PASS, AT LEAST THREE SUPPORT LEGS WHICH MAY PASS THROUGH

3 Sheets-Sheet 1 w. A. TAM

l OFFSHORE STORAGE STRUCTURE Feb, 2o, 1913 Filed Dec. 27, 1971 Feb. 20, 1973 w. A. TAM L 3,717,001

OFFSHORE STORAGE STRUCTURE Find Dec. 27.v 1971 3 Sheets-Sheng Feb. 2o, 1913 w, A, TAM 331mm OFFSHORE STORAGE STRUCTURE Filed Dec. 27, 1971 3 Sheets-Sheet 3 1 111: :tz/ 71;? 55g. "fl T -l 5 ----JZ United States Patent O 3,717,001 OFFSHORE STORAGE STRUCTURE William A. Tam, Warrenville, lll., assigner to Chicago Bridge & Iron Company, Oak Brook, lll. Filed Dec. 27, 1971, Ser. No. 212,224 Int. Cl. E02!) 21/00; B63b 35/44; B65g 87/00 U.S. Cl. til-46.5 8 Claims ABSTRACT OF THE DISCLOSURE Stable offshore storage structures including: a first, ringwall-reinforced tank disposable adjacent the sea oor; a vertically extending shaft centrally disposed Within the first tank and extending upwardly therefrom; a second, annularly-shaped tank disposable above the first tank, through the central orifice of which the shaft may pass; at least three support legs which may pass through leg retaining wells associated with both the ringwall and the second tank; and, means for securing and vertically moving the legs relative to the ringwall and second tank. There may also be provided means associated with the second tank for securing and vertically moving it relative to the shaft.

BACKGROUND OF THE INVENTION The present invention relates generally to offshore storage facilities and more particularly to structures suitable for storage of large quantities of oil at substantial depths in offshore locations.

The proliferation of offshore oil wells in recent years has given rise to an increased demand for facilities capable of storing large quantities of crude oil. A number of problems have consistently attended the construction, placement and use of offshore storage tanks, especially those having a large capacity (e.g., from about 750,000 to about 1,000,000 barrels) and those intended to be used at substantial depths (c g., from about 400 to about 600 or more feet).

It is ordinarily the practice, for example, that sizeable portions of the construction of undersea storage tanks are performed on site and, consequently, there is often a need on site for auxiliary equipment such as derricks, barges and the like. It is also often the case that a number of underwater divers are required to perform various subsurface construction operations. Further, extreme care must be exercised in selection of a site for positioning present offshore storage tanks since uneven sites are ordinarily unacceptable.

Lastly, piling is ordinarily required to secure offshore storage tanks to the sea oor. This is due principally to the unbalanced uplifting bouyant forces generated by the differences in specific gravity between oil stored in an undersea tank and the water surrounding such tanks.

SUMMARY OF THE INVENTION Briefly stated, offshore storage facilities constructed according to the present invention include first and second tanks, the first tank being a ringwall-reinforced storage structure disposable adjacent the sea floor and having vertically extending therefrom a hollowed shaft portion; the second tank being annularly-shaped and including means mounting it for securing and vertically moving it in relation to the shaft-portion of the first tank. Apertures are provided in the ringwall of the first tank and in the body of the second tank through which support legs may pass. Also provided are means mounting the support legs for securing and vertically moving them both relative to the ringwall of the first tank and to the annular tank.

Construction of offshore storage facilities of the present 3,717,001 Patented Feb. 20, 1973 ICC invention may be substantially completed at a graving dock away from their projected offshore locations with consequent minimization of the need for offshore construction work, auxiliary equipment and diving personnel.

Due to their double tank, over and under configuration, storage facilities of the invention are exceptionally stable structures, not ordinarily subject to uplift and cyclic loading while being filled and emptied. As such, they will accept an uneven site and will ordinarily not require piling. They are thus easily recoverable and may be moved from site to site as desired. Advantages in recoverability of the structures are further enhanced by their inherent suitability for use at offshore sites of widely varying water depths.

Further aspects and advantages of the present invention will become apparent upon consideration of the following detailed description of a preferred exemplary ernbodiment thereof along with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. l is a side elevational view of an embodiment of a storage facility of the invention in operating configuration at an offshore location with portions of the storage tank, ringwall and support legs cut away.

FIG. 2 is a side elevational view of a storage facility of the invention in a towing configuration.

FIG. 3 is a sectional view taken along lines 3 3 of FIG. 2.

FIGS. 4 through 7 are side elevational views of a storage facility in various configurations during positioning at an offshore location.

DETAILED DESCRIPTION FIG. l illustrates a storage structure 10 of the present invention including a hemispheric tank 11, such as those of the open-bottomed Dubai-type, having a circumferential ring wall 12 disposed about the perimeter of tank base portion 13. Ringwall 12 is preferably of reinforced concrete material.

Tank 11 is provided with a centrally located, hollowed, vertically extending shaft element 14 through which material to be stored may be passed in filling tank 11. As shown, shaft 14 is of a length sufficient to extend from the bottom portion of tank 11 to above the waters surface when the storage structure is in an operating configuration offshore.

A plurality of reinforcing struts 15 may be affixed both to base portion 16 of shaft 14 and base portion 1'3 of tank 11 to form a spider assembly (FIG. 3), allowing the tank to resist moments induced on shaft `14 by wave action and the like.

Ringwall 12 of the embodiment shown is provided with at least three leg retaining wells 17 or similar retaining structures through which at least three support leg elements 18 may pass. When three legs 18` are employed, wells 17 are preferably disposed at 120 intervals within or exteriorly of the ringwall. Legs 18 are preferably of a tubular shape and are of a length sufficient to extend from the sea Vfloor to above the water surface when the storage structure is in an operating configuration offshore.

Upper tank 19 is of annular shape, permitting passage of shaft 14 through central orice 20 thereof. Annular tank 19 is also provided with at least three wells 21, preferably disposed at intervals and spatially corresponding to wells 17 of ringwall 12, through which legs 18 may pass.

As shown in FIG. 2, pneumatic, hydraulic or mechanical jack and gripper elements 22 are associated with wells 17 of ringwall 12 and the wells 21 of annular tank 19 to provide means mounting support legs 18 for securing and vertically moving them both relative to ringwall 12 of tank 11 and to annular tank 19. Similarly, a jack and gripper element 23 may be provided at orifice 20 of annular tank 19 to provide means mounting tank 19 for securing and moving it relative to shaft 14.

J ack and gripper elements suitable for use in the structures of the present invention may be of the type disclosed in U.S. Pat. Nos. 2,775,869, 2,873,580, and 2,948,119.

When the storage facility is in operative configuration offshore it may be desirable to provide tie rods 24 as means for providing additional stability.

The dimensions of the elements of a storage structure of the invention may be subject to substantial variation. A typical structure, however, would include a tank 11 of about 850,000 barrel capacity. In such a case, shaft 14 would have a diameter of about thirty feet, legs 18 would have a diameter of about fifteen feet and annular tank 19 a height of about 20 feet.

FIGS. 2 and 4 show a storage structure 10 in towing configuration upon leaving the graving dock. Buoyancy of the structure is prinicpally provided by air trapped in tank 11. Legs 18 are raised out of the water to minimize drag. Annular tank 1-9 is at a position on shaft v14 above tank 11.

Upon reaching the desired offshore location, legs 18 are lowered or jacked (FIG. 5) toward the sea floor and gripped in place relative to annular tank 19 by jack and gripper elements 22 at wells 21. Air is then vented from tank 11 by conventional means (not shown). As air is so vented, tank 11 is jacked downward along legs 18 by jack and gripper elements 22 at ringwall wells 17. Shaft 14` meanwhile slides or is jacked downward through orifice 20 of annular tank 19 (FIG. 6). It will be noted that, as tank 11 is lowered, structure 10 is supported by the buoyancy of annular tank 19 and has pendulum-like stability. t

When some portion of tank 11 touches the sea floor, legs 18 are released from secured position relative to tank 19 and are individually jacked downward into' the sea oor as far as possible (YFIG. 7). Tank 19 is then jacked upward on legs 18- until it is raised above the water surface (FIG. 8). It is then gripped in secured position to legs 18 and filled with Water or other liquid in order to apply an additional downward load on legs 18 to push them still further into the sea floor (FIG. 9).

It will be appreciated that storage structures constructed and positioned in the manner described above will quite easily accept an uneven sea `tloor site since a stable configuration may be achieved without the entire ringwall substantially contacting the sea iloor.

In the event that an unstable intermediate configuration is attained in the course of positioning, the aboveoutlined steps may be reversed to the desired extent and the structure thereafter repositioned. Similarly, if it is desired to relocate the structure, recovery can be effected merely by reversal of the above steps.

'In the event that the structure is to be employed as a permanent storage installation, it may be desired that points of contact between the legs and the ringwall or annular tank be grout sealed and/or welded. Further, in such permanent installations, it may be desired to employ the upper surface of tank 19 as a drilling platform and to elect drilling through one or more of legs 18.

Obviously modilications and variations of the abovedescribed invention may be made without departing from the spirit and scope thereof. Por example, it may be de- 4 srable to provide more than three support legs, to vary the general shape of hemispheric tank 411, or to provide a closed-bottom tank together with standard means for iilling it with liquid as air is vented from it in the course of placement. Therefore, only such limitations as are indicated in the appended claims shall be placed thereon.

What is claimed is:

1. A stable olTshore storage structure comprising:

a first tank, said irst tank being ringwall-reinforced,

and disposable adjacent the sea iloor;

a vertically extending, shaft element associated with the central portion of said first tank and extending outwardly and upwardly therefrom;

a second tank disposed above said rst tank, said second tank being annularly-shaped and having a central orice through which said shaft may pass;

at least three support legs associated with said rst and second tanks;

at least three leg retaining well means associated with the ringwall of said first tank through which said support legs pass;

means associated with the well means of said ringwall for securing and vertically moving said legs relative to said ringwall;

at least three leg retaining Well means in said second tank through which said support legs may pass;

means associated with the well means of said second tank for securing and vertically moving said legs relative to said second tank;

whereby said iirst tank may be disposed adjacent the sea Hoor, said legs may be moved into the sea oor, and second tank may be positioned about said shaft and above the waters surface.

2. The storage structure of claim 1 wherein said means for securing and vertically moving said legs comprise pneumatic jack and gripper elements.

3. The storage structure of claim 1 further including means associated with said central orifice of said second tank for securing and vertically moving said second tank relative to said shaft.

4. The storage structure of claim 4 wherein said means for securing and vertically, moving said second tank comprise pneumatic jack and gripper elements.

5. The storage structure of claim 1 wherein said iirst tank is open-bottomed.

6. The storage structure of claim 1 wherein said shaft is hollowed and extends through the central portion of said first tank.

7. The storage structure of claim 6 further including reinforcing strut means associated with the base portion of said shaft and the base portion of said rst tank.

8. The storage structure of claim 1 wherein said support legs are hollow.

References Cited UNITED STATES PATENTS 2,771,747 ll/ 1956 Rechtin 61-46.5

3,429,128 271-969 Stafford et al. 61-46 FOREIGN PATENTS 1,196,992 7/ 1970 IGreat Britain 61-46.5

JACOfB SHAPIRO, `Primary Examiner 

